Microwave power transmission system wherein level of transmitted power is controlled by reflections from receiver

ABSTRACT

A microwave, wireless, power transmission system in which the transmitted power level is adjusted to correspond with power required at a remote receiving station in which deviations in power load produce an antenna impedance mismatch causing variations in energy reflected by the power receiving antenna employed by the receiving station. The variations in reflected energy are sensed by a receiving antenna at the transmitting station and used to control the output power of a power transmitter.

United States atent 1 1 Robinson, Jr.

[451 Mar. 5, 1974 MICROWAVE POWER TRANSMISSION SYSTEM WHEREIN LEVEL OFTRANSMITTED POWER IS CONTROLLED BY REFLECTIONS FROM RECEIVER [75]Inventor: William J. Robinson, Jr., Huntsville,

Ala,

[73] Assignee: The United States of America as represented by theAdministrator of the National Aeronautics and Space Administration,Washington, DC.

[22] Filed: Mar. 13, 1973 [21] Appl. No.: 340,871

[52] US. Cl 343/7.5, 325/62, 333/17, 343/177 [51] Int. Cl G01s 9/02,1104b 3/04 [58] Field of Search 343/7.5, 17.7, 705; 325/4, 325/62, 159,225; 333/17 [56] References Cited UNITED STATES PATENTS 3,271,679 9/1966Fostoff 325/62 X 3,535,543 10/1970 Dailey ..343/705X PrimaryExaminerPaul L. Gensler Attorney, Agent, or FirmL. D. Wofford, Jr.; G.J. Porter; J. R. Manning [5 7] ABSTRACT A microwave, wireless, powertransmission system in which the transmitted power level is adjusted tocorrespond with power required at a remote receiving station in whichdeviations in power load produce an antenna impedance mismatch causingvariations in energy reflected by the power receiving antenna employedby the receiving station. The variations in reflected energy are sensedby a receiving antenna at the transmitting station and used to controlthe output power of a power transmitter.

3 Claims, 4 Drawing Figures MICROWAVE TRANSMITTER l i VARIABLE 20VARIABLE POWER SUPP LY 23 POWER CONTROL E MICROWAVE POWER TRANSMISSIONSYSTEM WHEREIN LEVEL OF TRANSMITTED POWER IS CONTROLLED BY REFLECTIONSFROM RECEIVER ORIGIN OF THE INVENTION The invention described herein wasmade by an employee of the United States Government and may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to systems for the transmission of electrical power by microwaveradiant energy.

2. General Description of the Prior Art It has heretofore been proposedthat power be transmitted between otherwise inaccessible locations bymicrowave radio transmission. Such a method is particularly applicableto the transmission of power through space as between earth and a spacestation, between earth and the moon or between space stations. Of

7 course, the method is also applicable to transmission of power betweenlocations on earth where the cost or difficulty in building power linesmakes transmission by microwave energy feasible.

One difficulty with transmission of power by radio means, particularlywhere the receiving station is not manned, is that of transmittingcorrect levels of power, that is levels of power which are needed at aparticular time. This is a special problem where there are significantvariations in power load at a receiving station. Aside from the waste,problems arise in the dissipation of unneeded energy and in maintainingcorrect voltage levels.

The applicant is unaware of any previously known systems whicheffectively provide desired regulation of power between the transmittingand receiving station of such a system.

SUMMARY OF THE INVENTION Accordingly, it is the objectof this inventionto provide a microwave power transmission system in which transmittedpower is regulated in accordance with power demand and utilization at areceiving station.

These and other objects are accomplished in the present invention inwhich the transmitting station includes a receiving antenna adapted toreceive reflected or reradiated energy from the receiving antenna of thereceiving station. The reflected energy increases with non-utilizationof energy at the receiving station and variations in received-reradiatedenergy are detected at the transmitting station and employed to regulatethe power output of the transmitter of the transmitting station enablingpower to be maintained at a level in accordance with actual demands ofthe receiving station.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fullyunderstood by the following detailed description when consideredtogether with the accompanying drawings in which:

FIG. 1 is a schematic illustration of a microwave power transmissionsystem constructed in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, thereis shown in FIG. 1a

block diagram of a microwave system for transmitting electrical powerbetween a space transmitting station 10 and space sub-station orreceiving station 12 wherein transmitting antenna 14 and receivingantenna 16 are precisely aligned for a maximum transfer of energy andare maintained at a precise distance apart. Transmitting station 10employs a microwave transmitter 18 which is powered by variable orregulatable power supply 20. Power supply 20 is controlled by powercontrol 22 in response to a reflected signal received by auxiliaryreceiving antenna 23 indicative of the power needed by the receivingstation as represented by variable. load 25. This is accomplished asfollows.

Microwave energy from transmitter 18 is fed to transmitting antenna 14which is a narrow beam antenna such as a generally eliptical dish-shapedantenna. This antenna is configured to transmit a microwave beam towardreceiving antenna 12. Receiving-antenna 12 is adapted to receive thetransmitted power with maximum efficiency and in accordance with oneaspect of the invention would include an array of half wave dipoles 28supported by and spaced from reflector 30 with dimensions appropriate tocapture the maximum amount of electromagnetic energy from transmittingstation 12. A power transmission system utilizing this general type ofantenna is disclosed in U.S. Pat. No. 3,535,543.

Referring to FIG. 2, dipoles 28 of such an antenna are each terminatedat the center by a bridge rectifier 32 which converts radio frequency(R.F.) current to direct current. The outputs of rectifiers 32 are fedto a summing circuit 34 wherein the outputs are appropriately connectedin a series parallel circuit arrangement to match the impedance ofvariable impedance load 25 when load 25 is operating at a maximum powerlevel, that is in a minimum impedance condition.

Receiving antenna 23 is a small directive antenna, mounted near, butpositioned in an R.F. null region with respect to, transmitting antenna14 at transmitting station 10. It samples R.F. energy reflected byreceiving antenna 16. The output of antenna 23 is coupled to powercontrol 22 which is adapted to provide a calibrated output signal whichis a selected proportional function of the RF. energy received. Thisoutput provides a control signal input to variable power supply 20 whichfunctions to decrease power to transmitter 18, and thus transmittedpower, to predetermined values with increased values of reflectedenergy. In this manner selected equilibrium conditions are achievedbetween transmitted power and load of variable load 25, and desiredamounts of power supplied to variable load 25 for selected discrete loadvalues.

Variable load 25 is typically representative of a combination of powereddevices connected electrically in parallel which may all be operated atone time to provide a minimum impedance load or one or more of thedevices may be disconnected, or turned off, to provide differing valuesof higher impedance. The object of this invention is, as indicatedabove, to transmit less power as fewer devices are being powered, whichis signified by increases in load impedance. In this system the quantityof energy reflected at the lowest selected transmitted power (highestimpedance-maximum antenna mismatch combination) is greater than withmaximum transmitter power with a matched or minimum impedance condition.Thus it is possible to provide a control system which is calibrated interms of a discrete value of reflected signal for a given load and powerlevel of transmitter 18.

FIG. 3 shows a simplified illustration of a circuit for power control22. Detector 36 receives R.F. energy from antenna 23 and provides a DC.output which is fed through limiter 38 to the plus input of comparator39. A reference voltage, e.g. 5 volts, is connected across potentiometer40 and to the plus input of subtraction or difference circuit 42. Theoutput of the power control appears on moveable arm 44 of potentiometer40 and is applied to variable power supply and to the minus output ofsubtraction circuit 42. The output of the subtraction circuit isconnected to the minus input of comparator 39 and the output of thecomparator drives reversible arm 44 to thus determine circuit output.Limiter 38, connected between detector 36 and comparator 39, limits theamplitude of signals applied to the comparator to a maximum calibratedoutput of detector 36 which is equal to the reference voltage.

FIG. 4 shows a graph of detector output voltage plot ted versusimpedance load conditions. Curve 48 extends between selected operatinglimits, being between minimum voltage point 50, which is representativeof reflected energy with maximum power and a matched impedance load, anda maximum voltage point 52 which is representative of reflected energywith a minimum power but with a maximum impedance, and thus with amaximum impedance mismatch. The output of detector 36 is calibrated interms of curve 48 which indicates a selected power output for each loadvalue.

To examine operation of the system, it is initially assumed that thesystem is in equilibrium wherein transmitter 18 is operating withmaximum power, e.g. 500 watts, to provide a desired level of power tovariable load which is initially providing a matched impedance toreceiving antenna 16, e.g. 20 ohms, as shown in FIG. 4. This, as will benoted, is the minimum operating impedance value of load 25. It isfurther assumed that power supply 20 provides a level of power totransmitter 18 to achieve maximum power output with a +4 volt input fromarm 44 of potentiometer and with control arm 44 in the position shown inFIG. 4. It is further assumed that the reflected energy from receivingantenna 16 picked up by antenna 23, and detected by detector 36,provides detector output of +1 volt. Thus, there will be applied to theplus input of comparator 39 a +1 volt. The +4 volt output of control arm44 is applied to the minus input of subtraction circuit 42 in which thisvalue is subtracted from a reference +5 volts to provide a +1 volt tothe minus input of comparator 39. Thus, initially comparator 39 hasequal voltages applied and it provides no output to motor 46 and thuscontrol arm 44 is left at rest and the power level of transmitter 18 isnot changed.

It is next assumed that variable load 25 increases in impedance to avalue of, e.g. 45 ohms, indicating a decrease in need of power, and fromcurve 48 it will be noted that this has been fixed at 400 watts. Therewill then occur an increase in reflected power from antenna 16 which issensed by antenna 23, causing the output of detector 36 to rise to somevalue above curve 48, e.g. +3 volts as represented by point 54. This +3volts is applied to the plus input of comparator 39 and with the +1 voltof the minus terminal there is now a net +2 volts applied to thecomparator resulting in a positive output voltage. Motor 46 is thencaused to rotate in a direction to cause arm 44 to move downward. As itdoes, the resulting decrease output voltage of potentiometer 40 is fedto variable power supply 20 reducing the power output of transmitter 18and power supplied receiving antenna 116. This in turn results in adecrease in reflected power back to antenna 23 and to a reduced outputof detector 36. Thus there will occur a decrease in input to the plusinput of comparator 39 and an increase in input to the minus input. Thisoccurs since the power supply control voltage on arm 44 decreases,representative of the power output of transmitter 18, and this voltageis subtracted from the +5 volts reference voltage. When the voltagesapplied to comparator 39 are equal, which will occur at intersectionpoint 56 on curve 48, the output of comparator 39 will again be madezero, turning off motor 46 and a new desired power level will have beenachieved.

Since detector 36 is appropriately compensated and calibrated asillustrated by curve 48 to provide an output which varies generally inproportion to load for each power setting, this means that between theoperating limits of the system, that for each load value there exists acoordinate power level. If the power level is too high, the outputvoltage of detector 36 will be above that indicated by curve 48 andwhenever there is insufficient power for a given load, the outputvoltage of detector 36 will be below curve 48. If the detector outputfor a given load is below curve 48, the net error voltage applied tocomparator 39 will be negative and motor 46 will be turned in adirection to cause arm 44 to provide a higher voltage and to cause powersupply 20 to effect a higher output from transmitter 18. An oppositepolarity output of detector 36 produces an opposite effect, to lowertransmitter power.

It is to be appreciated that the control circuit of power control 22illustrated in FIG. 3 is illustrative of only one circuit for thispurpose and that various systems of adjusting the power of transmitter118 responsive to the reflected energy from receiving antenna 16 may beemployed to provide a particular power output of transmitter 18 for aparticular load condition.

What is claimed is:

ll. A microwave power transmission system comprising:

a microwave power generator;

power supply means responsive to an input control signal for providingvariable operating power to said microwave power generator whereby saidmicrowave power generator is caused to provide an output of a selectedlevel of microwave energy;

a power transmitting antenna adapted to receive power from saidmicrowave power generator and direct microwave energy in an energy beamin a selected direction;

a power receiving antenna spaced from said power transmitting antennaand being positioned and oriented to receive maximum energy from saidtransmitting antenna;

a variable impedance, electrical, load connected to receive and useenergy from said power receiving antenna;

a second power receiving antenna positioned near said transmittingantenna and adapted to receive electrical energy reflected from saidfirst power receiving antenna;

control means responsive to the output of said second power antenna forproviding said control signal to said power supply means;

whereby said power supply means is controlled and the output of saidmicrowave power generator coordinately controlled in accordance withpower requirements of said variable impedance load.

2. A microwave power transmission system as set forth in claim 1 whereinsaid control means is operable for providing variable control signalsresponsive to load values variably between a load value substantiallyequal to the impedance of said power receiving antenna and a selectedimpedance value greater than the impedance of said power receivingantenna.

3. A microwave power transmission system as set forth in claim 2 whereinsaid control means includes means responsive to selected combinations ofload impedance-power levels of the system for providing said controlsignal and selected transmitter outputs for selected values of saidvariable impedance load.

1. A microwave power transmission system comprising: a microwave powergenerator; power supply means responsive to an input control signal forproviding variable operating power to said microwave power generatorwhereby said microwave power generator is caused to provide an output ofa selected level of microwave energy; a power transmitting antennaadapted to receive power from said microwave power generator and directmicrowave energy in an energy beam in a selected direction; a powerreceiving antenna spaced from said power transmitting antenna and beingpositioned and oriented to receive maximum energy from said transmittingantenna; a variable impedance, electrical, load connected to receive anduse energy from said power receiving antenna; a second power receivingantenna positioned near said transmitting antenna and adapted to receiveelectrical energy reflected from said first power receiving antenna;control means responsive to the output of said second power antenna forproviding said control signal to said power supply means; whereby saidpower supply means is controlled and the output of said microwave powergenerator coordinately controlled in accordance with power requirementsof said variable impedance load.
 2. A microwave power transmissionsystem as set forth in claim 1 wherein said control means is operablefor providing variable control signals responsive to load valuesvariably between a load value substantially equal to the impedance ofsaid power receiving antenna and a selected impedance value greater thanthe impedance of said power receiving antenna.
 3. A microwave powertransmission system as set forth in claim 2 wherein said control meansincludes means responsive to selected combinations of loadimpedance-power levels of the system for providing said control signaland selected transmitter outputs for selected values of said variableimpedance load.